(1) Field of the Invention
This invention relates to the casting of metals, particularly (although not exclusively) aluminum and aluminum alloys. More particularly, the invention relates to the co-casting of metal layers by direct chill casting techniques.
(2) Description of the Related Art
Metal ingots are commonly produced by direct chill (DC) casting of molten metals. This involves pouring a molten metal into a mold having cooled walls, an open upper end and (after start-up) an open lower end. The metal emerges from the lower end of the mold as a solid metal ingot that descends and elongates as the casting operation proceeds. Such casting techniques are particularly suited for the casting of aluminum and aluminum alloys, but may be employed for other metals too.
Casting techniques of this kind are discussed extensively in U.S. Pat. No. 6,260,602 to Wagstaff, issued on Jul. 17, 2001, which relates exclusively to the casting of monolithic ingots, i.e. ingots made of the same metal throughout and cast as a single layer. It is also known to cast multiple layers of metal in DC casting apparatus. This involves the use of a divider of some kind within the casting mold to create two or more compartments for different metal pools that form different metal layers in the cast ingot. The divider may be a thin metal sheet that is fed continuously into the mold as the casting commences and which becomes incorporated into the cast ingot, or the divider may be a relatively short fixed element or divider wall that remains in place in the entrance to the mold and separates the metals until they are sufficiently solid to contact each other without comingling of the molten metals. Apparatus of the former kind (movable divider) is disclosed, for example, in U.S. Pat. No. 6,705,384 issued on Mar. 16, 2004 to Kilmer et al. (the disclosure of which is incorporated herein by reference). Apparatus of the latter kind (fixed divider wall) may involve simultaneous co-casting of two or more layers or sequential co-casting in which the divider wall is generally cooled. Apparatus for sequential co-casting is disclosed, for example, in U.S. Patent Publication No. 2005/0011630 A1, published on Jan. 20, 2005 in the name of Anderson et al. (the disclosure of which is incorporated herein by reference). Sequential solidification involves the casting of a first layer (e.g. a layer intended as an inner layer or core) and then, subsequently but in the same casting operation, casting one or more layers of other metals (e.g. as cladding layers) on the first layer once it has achieved a suitable degree of solidification.
While these techniques are effective and successful, there is a continuing interest in improving the quality of the cast ingot and, especially, the strength and integrity of the interfacial bond between adjacent layers or between such layers and a divider incorporated into the cast ingot. If the interfacial bond is weak or compromised, layer separation may take place during casting or subsequent rolling of the ingot, or “blisters” may form during ingot annealing. Furthermore, there is also a continuing interest in avoiding the formation of cracks in the outer surface of the cast ingot produced in these ways.